Modern fabrication processes for producing semiconductor devices, such as integrated circuits, have long employed photolithography for transferring circuit patterns onto a semiconductor substrate, such as a wafer. In general, photolithography involves the performance of a sequence of process steps, including coating a semiconductor wafer with a resist layer, exposing the coated wafer to a patterned light source, developing the resist layer, processing the semiconductor wafer through the developed resist layer, and removing the resist layer. An optical photolithography stepper apparatus, sometimes referred to as a "step and repeat" or "stepper", is typically used to expose the resist layer. An image of each layer of an IC die is formed on a small rectangular piece of glass referred to as a reticle or mask. The mask or reticle is placed on the stepper and a reduced image thereof is projected onto a portion of the resist layer covering the semiconductor wafer. Specifically, the reticle patterns are transferred to the wafer by scanning the patterns through a narrow illumination slit.
When numerous IC's are to be fabricated from a single wafer, a mask used in the fabrication of any one IC is also used in the fabrication of the other IC's from the wafer. This is accomplished by using the stepper to index or "step" the wafer under an optical system which includes the mask or reticle. At each step, the photoresist is exposed to the optical system, typically with ultraviolet light, to form an aerial image of the mask on the layer of photoresist. The wafer is then removed from the stepper and the image is developed. At that point, the wafer is etched to remove portions of the underlying film, following which the wafer is ready for the next stage of processing, which might include for example, ion implantation, deposition or other types of etching processes. At a later stage in the fabrication process, the wafer is returned to the stepper for exposure of the wafer to a different mask.
The reticle or mask is composed of a glass substrate, such as quartz, on which there is formed a circuit pattern composed of materials such as chromium which prevents ultraviolet light from transmitting therethrough. The reticle is set in the stepper in order to expose a semiconductor wafer to light, and the circuit pattern formed on the reticle is imaged by the stepper onto the semiconductor substrate.
Semiconductor manufacturing processes are aimed at achieving up to 0.25 micron resolution in a high production environment. This goal is being driven by the need to develop competitive device performance and lower manufacturing costs per device. In order to increase the field size and improve critical dimensional control below 0.25 micron resolution, improvements in step and repeat technology will play a critical role. Improvements in the area of highly controllable, precise light sources, such as excimer lasers with appropriate dose control are important for solving illumination control problems and achieving exceptionally short exposure times.
In order to more precisely control the dose of light radiation projected on the wafer, the illumination system and scanner slit have, in the past, been provided with an adjustment that allows focusing of the image applied to the wafer. This adjustment system relies on movement and adjustment of mechanical elements, and particularly the displacement of the mechanical slit relative to the illumination source. As a result of the dependency on this mechanical adjustment, repeatable results are not always obtained from batch to batch since adjustment settings may change for a number of reasons. Moreover, the need to perform periodic preventive maintenance on equipment introduces the further possibility that adjustment settings may be inadvertently altered, thus making repeatable, precise dosage control impossible.
A typical step-and-repeat projection system carried out in a stepper is shown in FIG. 1. In the projection system 10, the image size is projected at 1:1 or reduced in size by 2.times. to 5.times.. Reducing the image size provides the benefit that the features on the reticle do not have to be as small as the final image and are therefore substantially easier to fabricate. Another benefit is that mask defects and imperfections are also reduced in size and thus become less severe. However, as the wafer size increases, i.e., to 200 mm or larger, it becomes impossible to design optical components that can project a mask over an entire wafer. Instead, refractive optical systems are designed to project an image over a small portion of the wafer and then step-and-repeat the complete wafer surface. In a typical projection system 10, an excimer laser or a high intensity mercury source 12 is used as the light source which is reflected by a mirror 14 onto a filter 16 and a condenser lens 18. The condenser lens 18 (or the collimating lens) focus the illumination on a reticle or mask 22, and then on a reduction lens 52 to image the mask onto the surface of a wafer 54. After each exposure, the wafer is mechanically stepped a predetermined distance, realigned and refocused before the image is projected again in the step-and-repeat manner.
Referring now to FIG. 2 which depict a prior art photolithography illumination system and related dosage control technique. Radiation in the form of ultraviolet light is produced by a lamp 20 and is focused by a reflector 24 onto a reflecting mirror 26 to produce a beam of light which is sequentially passed through a filter 28, shutter 30, attenuator 32, zoom lens 34, interference filters 36, integrator lens 38 and field lens 40 onto an aluminized mirror 44. A portion of the beam focused onto the mirror 44 passes therethrough onto an energy sensor 46 which measures the intensity of the beam. The remainder of the beam is reflected by the mirror 44 as a beam spot 48 which is directed through reticle masking blades 50 defining a slit 51 thence through a reticle or mask (not shown). Radiation in the beam passes through the slit 51 and circuit pattern defining reticle and is focused onto a semiconductor wafer 54 that is supported on an exposure chuck 52 provided with a spot meter 56 for measuring the intensity of the beam focused onto a portion of the wafer 54.
The shutter 30 in FIG. 2 provides for exposure control by allowing a predetermined amount of UV light onto the photomask and thus producing an image on the wafer. Conventionally, the shutter is a mechanical device which is not only too slow to control, but also inaccurate in achieving precision control. For instance, the best accuracy that can be obtained by a mechanical shutter device is about 0.5 mm which is insufficient for exposure control in producing high density devices.
It is therefore an object of the present invention to provide an apparatus for controlling light transmission that does not have the drawbacks or shortcomings of a conventional shutter.
It is another object of the present invention to provide an apparatus for controlling light transmission in stepper exposure for a photolithographic process.
It is a further object of the present invention to provide an apparatus for stepper exposure control by utilizing a shutter that operates electronically.
It is another further object of the present invention to provide an apparatus for stepper exposure control in a photolithographic process that utilizes electro-optic crystal materials.
It is still another object of the present invention to provide an apparatus for stepper exposure control in a photolithographic process by utilizing four layers of electro-optic crystal materials wherein two layers are arranged in a parallel direction and two layers are arranged in a perpendicular direction.
It is yet another object of the present invention to provide an apparatus for stepper exposure control in a photolithographic process by utilizing shutters formed of LiNbO.sub.3 electro-optic crystal.
It is still another further object of the present invention to provide a shutter for exposure control in a stepper used in photolithography that includes light transmission control units which are formed of a multiplicity of parallel, spaced-apart lines of an electro-optic crystal embedded in a polarizer material on a quartz plate.
It is yet another further object of the present invention to provide a method for controlling light transmission by first providing a shutter formed of an electro-optic crystal material that stops light transmittance when the crystal material is charged with electricity.